On industrial sites, there are different types of installations in which fluids of various types circulate.
These installations comprise pipes in which fluids circulate and can likewise comprise reactors such as, for example, heat exchangers.
In this exact case, the fouling of such installations can have adverse effects to the degree it is capable of affecting the performances of the installations (for example, the efficiency of an industrial process).
Moreover, when fouling forms on the inside wall of a pipe or a reactor, it should be promptly cleaned.
However, it is necessary that this fouling be continuously detectable by the operators or the maintenance personnel of the installation in order to be able to assess, within the framework of preventive maintenance, the best time for cleaning.
For whatever reason, the fouling irregularly causes a shutdown of the installation during a sometimes indeterminate interval; this has serious adverse effects on the progression of the industrial process.
These interventions can represent tedious tasks for personnel, the more so if the fouling has only been detected belatedly and if its thickness is too great.
This removal of fouling has a not inconsiderable economic cost since the cost of a temporary shutdown of operation should be included in the cost of maintenance operations.
It should likewise be noted that as the heat exchangers become fouled, there is a progressive loss of efficiency before a potential shutdown of operation of the installation or the part of the installation comprising these exchangers.
Moreover, in hot water sanitation networks and in open industrial air-cooled towers, bacteria can develop within the network and the cooling circuit.
Likewise, a risk of contamination by Legionnaire's disease can be envisioned.
Currently, there should be regular monitoring of the installation by anticipating points of attack in the pipes or in the reactors in which the fluids that can cause fouling are circulating.
These points of attack likewise allow the taking of samples, and then the laboratory analysis of said samples to obtain either a measurement of fouling or an analysis of the type of fouling that has formed (type, composition . . . ).
On certain industrial sites, to measure the thickness of the layer of fouling that has formed within the walls of a pipe or a reactor, methods are used that require measurement of the loss of load that is produced between two points that are spaced in the direction of fluid flow. Likewise, methods that measure the temperature differences between these points can be used.
These latter measurements, however, present genuine problems to the degree in which:                They do not allow local information to be obtained,        They lack reactivity, but likewise sensitivity and extent of the measurement range.        
Document FR 2 885 694 discloses a method for measuring the fouling in a reactor or a pipe that uses two temperature probes.
More particularly, these two probes are introduced into a pipe respectively based on two points of attack, and one of these probes measures the temperature of the fluid while the other probe measures the temperature of the wall of a heat generator.
According to this method, the point is first of all to obtain a temperature difference between the wall temperature and the fluid temperature that is as near zero as possible. Then, the heat generator emits a heat flow while the temperature deviation between the wall temperature and that of the fluid is measured over time, the state of fouling of the reactor being determined based on the measurement of this temperature deviation.
This method and the associated system, however, have certain defects that limit their use in an industrial environment.
In particular, the presence of two points of physical attack on one pipe or a reactor always constitutes an installation constraint for a manufacturer, accompanied by a not inconsiderable cost.
Moreover, two temperature probes, even if they are of the same type, always have a certain drift of operation relative to one another due to, for example, variances that arise during their manufacture.
Because of these drifts, the two probes do not have the same behavior relative to one another vis-a-vis the same temperature of the environment into which they are immersed.
Moreover, the temperature probe that is used as a reference (the one that measures the fluid temperature) can itself become fouled; this introduces an additional drift relative to the other temperature probe.
Due likewise to the kinetic (or dynamic) differences of responses between the two temperature probes, a temperature deviation between the two probes can then be noted, whereas theoretically such a temperature deviation should not arise.
Then, the method used in the aforementioned document dictates the complete absence of variation of the temperature of the fluid into which the two separate temperature measuring elements are immersed. This is because this significantly reduces the range of applications to the degree in which most industrial processes and/or water treatment processes continuously modify and perturb the average temperature of the environment.
Finally, the method used, by imposing initial conditions, at the same time requires a posteriori processing of the recorded data as well as systematic verification of the conditions before any use. Thus, this makes this method useless for continuous applications or for long-term operation (24 h/24). At best, the access to the temperature deviation (thermal drift) can be observed over the anticipated and programmed measurement period.
The defects that were just cited can thus lead to faulty measurements of fouling and thus to a lack of reliability of the method used. Moreover, as a result of the operating mode and constituent elements of the physical device, the number of possible applications is limited.